Problem: On the first 3 exams of his chemistry class, Michael got an average score of 79. What does he need on the next exam to have an overall average of 83?
Let his score on the next exam be $x$ The sum of all of his scores is then $3 \cdot 79 + x$ The same sum must also be equal to $4 \cdot 83$ Solve: $x = 4 \cdot 83 - 3 \cdot 79 = 95$.